Systematic error mitigation in multiple field astrometry

Combination of more than two fields provides constraints on the systematic error of simultaneous observations. The concept is investigated in the context of the Gravitation Astrometric Measurement Experiment (GAME), which aims at measurement of the PPN parameter $\gamma$ at the $10^{-7}-10^{-8}$ level. Robust self-calibration and control of systematic error is crucial to the achievement of the precision goal. The present work is focused on the concept investigation and practical implementation strategy of systematic error control over four simultaneously observed fields, implementing a "double differential" measurement technique. Some basic requirements on geometry, observing and calibration strategy are derived, discussing the fundamental characteristics of the proposed concept.